The following section is included to inform users about the potential risks,
whether known or unknown, of using this tool. The two main categories of risks
are those created by the nature of the tool (e.g. read-only tools vs. read-write
tools) and those created by bugs.

pt-variable-advisor reads MySQL’s configuration and examines it and is thus
very low risk.

At the time of this release, we know of no bugs that could cause serious harm to
users.

The authoritative source for updated information is always the online issue
tracking system. Issues that affect this tool will be marked as such. You can
see a list of such issues at the following URL:
http://www.percona.com/bugs/pt-variable-advisor.

pt-variable-advisor examines SHOWVARIABLES for bad values and settings
according to the “RULES” described below. It reports on variables that
match the rules, so you can find bad settings in your MySQL server.

At the time of this release, pt-variable-advisor only examples
SHOWVARIABLES, but other input sources are planned like SHOWSTATUS
and SHOWSLAVESTATUS.

These are the rules that pt-variable-advisor will apply to SHOW VARIABLES.
Each rule has three parts: an ID, a severity, and a description.

The rule’s ID is a short, unique name for the rule. It usually relates
to the variable that the rule examines. If a variable is examined by
several rules, then the rules’ IDs are numbered like “-1”, “-2”, “-N”.

The rule’s severity is an indication of how important it is that this
rule matched a query. We use NOTE, WARN, and CRIT to denote these
levels.

The rule’s description is a textual, human-readable explanation of
what it means when a variable matches this rule. Depending on the
verbosity of the report you generate, you will see more of the text in
the description. By default, you’ll see only the first sentence,
which is sort of a terse synopsis of the rule’s meaning. At a higher
verbosity, you’ll see subsequent sentences.

auto_increment

severity: note

Are you trying to write to more than one server in a dual-master or
ring replication configuration? This is potentially very dangerous and in
most cases is a serious mistake. Most people’s reasons for doing this are
actually not valid at all.

concurrent_insert

severity: note

Holes (spaces left by deletes) in MyISAM tables might never be
reused.

connect_timeout

severity: note

A large value of this setting can create a denial of service
vulnerability.

debug

severity: crit

Servers built with debugging capability should not be used in
production because of the large performance impact.

delay_key_write

severity: warn

MyISAM index blocks are never flushed until necessary. If there is
a server crash, data corruption on MyISAM tables can be much worse than
usual.

flush

severity: warn

This option might decrease performance greatly.

flush_time

severity: warn

This option might decrease performance greatly.

have_bdb

severity: note

The BDB engine is deprecated. If you aren’t using it, you should
disable it with the skip_bdb option.

init_connect

severity: note

The init_connect option is enabled on this server.

init_file

severity: note

The init_file option is enabled on this server.

init_slave

severity: note

The init_slave option is enabled on this server.

innodb_additional_mem_pool_size

severity: warn

This variable generally doesn’t need to be larger than 20MB.

innodb_buffer_pool_size

severity: warn

The InnoDB buffer pool size is unconfigured. In a production
environment it should always be configured explicitly, and the default
10MB size is not good.

innodb_checksums

severity: warn

InnoDB checksums are disabled. Your data is not protected from
hardware corruption or other errors!

innodb_doublewrite

severity: warn

InnoDB doublewrite is disabled. Unless you use a filesystem that
protects against partial page writes, your data is not safe!

innodb_fast_shutdown

severity: warn

InnoDB’s shutdown behavior is not the default. This can lead to
poor performance, or the need to perform crash recovery upon startup.

innodb_flush_log_at_trx_commit-1

severity: warn

InnoDB is not configured in strictly ACID mode. If there
is a crash, some transactions can be lost.

innodb_flush_log_at_trx_commit-2

severity: warn

Setting innodb_flush_log_at_trx_commit to 0 has no performance
benefits over setting it to 2, and more types of data loss are possible.
If you are trying to change it from 1 for performance reasons, you should
set it to 2 instead of 0.

innodb_force_recovery

severity: warn

InnoDB is in forced recovery mode! This should be used only
temporarily when recovering from data corruption or other bugs, not for
normal usage.

innodb_lock_wait_timeout

severity: warn

This option has an unusually long value, which can cause
system overload if locks are not being released.

innodb_log_buffer_size

severity: warn

The InnoDB log buffer size generally should not be set larger than
16MB. If you are doing large BLOB operations, InnoDB is not really a good
choice of engines anyway.

innodb_log_file_size

severity: warn

The InnoDB log file size is set to its default value, which is not
usable on production systems.

innodb_max_dirty_pages_pct

severity: note

The innodb_max_dirty_pages_pct is lower than the default. This can
cause overly aggressive flushing and add load to the I/O system.

flush_time

severity: warn

This setting is likely to cause very bad performance every
flush_time seconds.

key_buffer_size

severity: warn

The key buffer size is unconfigured. In a production
environment it should always be configured explicitly, and the default
8MB size is not good.

large_pages

severity: note

Large pages are enabled.

locked_in_memory

severity: note

The server is locked in memory with –memlock.

log_warnings-1

severity: note

Log_warnings is disabled, so unusual events such as statements
unsafe for replication and aborted connections will not be logged to the
error log.

log_warnings-2

severity: note

Log_warnings must be set greater than 1 to log unusual events such
as aborted connections.

low_priority_updates

severity: note

The server is running with non-default lock priority for updates.
This could cause update queries to wait unexpectedly for read queries.

max_binlog_size

severity: note

The max_binlog_size is smaller than the default of 1GB.

max_connect_errors

severity: note

max_connect_errors should probably be set as large as your platform
allows.

max_connections

severity: warn

If the server ever really has more than a thousand threads running,
then the system is likely to spend more time scheduling threads than
really doing useful work. This variable’s value should be considered in
light of your workload.

myisam_repair_threads

severity: note

myisam_repair_threads > 1 enables multi-threaded repair, which is
relatively untested and is still listed as beta-quality code in the
official documentation.

old_passwords

severity: warn

Old-style passwords are insecure. They are sent in plain text
across the wire.

optimizer_prune_level

severity: warn

The optimizer will use an exhaustive search when planning complex
queries, which can cause the planning process to take a long time.

port

severity: note

The server is listening on a non-default port.

query_cache_size-1

severity: note

The query cache does not scale to large sizes and can cause unstable
performance when larger than 128MB, especially on multi-core machines.

query_cache_size-2

severity: warn

The query cache can cause severe performance problems when it is
larger than 256MB, especially on multi-core machines.

read_buffer_size-1

severity: note

The read_buffer_size variable should generally be left at its
default unless an expert determines it is necessary to change it.

read_buffer_size-2

severity: warn

The read_buffer_size variable should not be larger than 8MB. It
should generally be left at its default unless an expert determines it is
necessary to change it. Making it larger than 2MB can hurt performance
significantly, and can make the server crash, swap to death, or just
become extremely unstable.

read_rnd_buffer_size-1

severity: note

The read_rnd_buffer_size variable should generally be left at its
default unless an expert determines it is necessary to change it.

read_rnd_buffer_size-2

severity: warn

The read_rnd_buffer_size variable should not be larger than 4M. It
should generally be left at its default unless an expert determines it is
necessary to change it.

relay_log_space_limit

severity: warn

Setting relay_log_space_limit can cause replicas to stop fetching binary logs
from their master immediately. This could increase the risk that your data will
be lost if the master crashes. If the replicas have encountered a limit on relay
log space, then it is possible that the latest transactions exist only on the
master and no replica has retrieved them.

slave_net_timeout

severity: warn

This variable is set too high. This is too long to wait before
noticing that the connection to the master has failed and retrying. This
should probably be set to 60 seconds or less. It is also a good idea to
use pt-heartbeat to ensure that the connection does not appear to time out
when the master is simply idle.

slave_skip_errors

severity: crit

You should not set this option. If replication is having errors,
you need to find and resolve the cause of that; it is likely that your
slave’s data is different from the master. You can find out with
pt-table-checksum.

sort_buffer_size-1

severity: note

The sort_buffer_size variable should generally be left at its
default unless an expert determines it is necessary to change it.

sort_buffer_size-2

severity: note

The sort_buffer_size variable should generally be left at its
default unless an expert determines it is necessary to change it. Making
it larger than a few MB can hurt performance significantly, and can make
the server crash, swap to death, or just become extremely unstable.

sql_notes

severity: note

This server is configured not to log Note level warnings to the
error log.

sync_frm

severity: warn

It is best to set sync_frm so that .frm files are flushed safely to
disk in case of a server crash.

tx_isolation-1

severity: note

This server’s transaction isolation level is non-default.

tx_isolation-2

severity: warn

Most applications should use the default REPEATABLE-READ transaction
isolation level, or in a few cases READ-COMMITTED.

expire_log_days

severity: warn

Binary logs are enabled, but automatic purging is not enabled. If
you do not purge binary logs, your disk will fill up. If you delete
binary logs externally to MySQL, you will cause unwanted behaviors.
Always ask MySQL to purge obsolete logs, never delete them externally.

innodb_file_io_threads

severity: note

This option is useless except on Windows.

innodb_data_file_path

severity: note

Auto-extending InnoDB files can consume a lot of disk space that is
very difficult to reclaim later. Some people prefer to set
innodb_file_per_table and allocate a fixed-size file for ibdata1.

innodb_flush_method

severity: note

Most production database servers that use InnoDB should set
innodb_flush_method to O_DIRECT to avoid double-buffering, unless the I/O
system is very low performance.

innodb_locks_unsafe_for_binlog

severity: warn

This option makes point-in-time recovery from binary logs, and
replication, untrustworthy if statement-based logging is used.

innodb_support_xa

severity: warn

MySQL’s internal XA transaction support between InnoDB and the
binary log is disabled. The binary log might not match InnoDB’s state
after crash recovery, and replication might drift out of sync due to
out-of-order statements in the binary log.

log_bin

severity: warn

Binary logging is disabled, so point-in-time recovery and
replication are not possible.

log_output

severity: warn

Directing log output to tables has a high performance impact.

max_relay_log_size

severity: note

A custom max_relay_log_size is defined.

myisam_recover_options

severity: warn

myisam_recover_options should be set to some value such as
BACKUP,FORCE to ensure that table corruption is noticed.

storage_engine

severity: note

The server is using a non-standard storage engine as default.

sync_binlog

severity: warn

Binary logging is enabled, but sync_binlog isn’t configured so that
every transaction is flushed to the binary log for durability.

tmp_table_size

severity: note

The effective minimum size of in-memory implicit temporary tables
used internally during query execution is min(tmp_table_size,
max_heap_table_size), so max_heap_table_size should be at least as large
as tmp_table_size.

old mysql version

severity: warn

These are the recommended minimum version for each major release: 3.23, 4.1.20, 5.0.37, 5.1.30.

Default character set. If the value is utf8, sets Perl’s binmode on
STDOUT to utf8, passes the mysql_enable_utf8 option to DBD::mysql, and
runs SET NAMES UTF8 after connecting to MySQL. Any other value sets
binmode on STDOUT without the utf8 layer, and runs SET NAMES after
connecting to MySQL.

Create the given PID file when daemonized. The file contains the process
ID of the daemonized instance. The PID file is removed when the
daemonized instance exits. The program checks for the existence of the
PID file when starting; if it exists and the process with the matching PID
exists, the program exits.

These DSN options are used to create a DSN. Each option is given like
option=value. The options are case-sensitive, so P and p are not the
same option. There cannot be whitespace before or after the = and
if the value contains whitespace it must be quoted. DSN options are
comma-separated. See the percona-toolkit manpage for full details.

This tool is part of Percona Toolkit, a collection of advanced command-line
tools developed by Percona for MySQL support and consulting. Percona Toolkit
was forked from two projects in June, 2011: Maatkit and Aspersa. Those
projects were created by Baron Schwartz and developed primarily by him and
Daniel Nichter, both of whom are employed by Percona. Visit
http://www.percona.com/software/ for more software developed by Percona.

This program is copyright 2010-2012 Percona Inc.
Feedback and improvements are welcome.

THIS PROGRAM IS PROVIDED “AS IS” AND WITHOUT ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.

This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free Software
Foundation, version 2; OR the Perl Artistic License. On UNIX and similar
systems, you can issue `man perlgpl’ or `man perlartistic’ to read these
licenses.

You should have received a copy of the GNU General Public License along with
this program; if not, write to the Free Software Foundation, Inc., 59 Temple
Place, Suite 330, Boston, MA 02111-1307 USA.

This documentation is developed in Launchpad as part of the Percona Toolkit source code.
If you spotted innacuracies, errors, don't understood it or you think something is missing or should be improved, please file a bug.